Radio frequency (RF) transmitters are found in many one-way and two-way communication devices, such as portable communication devices (cellular telephones), personal digital assistants (PDAs) and other communication devices. An RF transmitter transmits using whatever communication methodology is dictated by the particular communication system within which it is operating. For example, communication methodologies typically include amplitude modulation, frequency modulation, phase modulation, or a combination of these. In a typical global system for mobile communications (GSM) communication system using narrowband TDMA technology, a GMSK modulation scheme supplies a low noise phase modulated (PM) transmit signal to a non-linear power amplifier directly from an oscillator.
In such an arrangement, a non-linear power amplifier, which is highly efficient, can be used, thus allowing efficient transmission of the phase-modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized.
One manner of controlling the output of the power amplifier is referred to as a closed-loop power control system. In a closed-loop power control system, a portion of the output of the power amplifier is diverted to closed-loop power control circuitry associated with the power amplifier. The closed-loop power control circuitry analyzes a number of factors, including a power control signal generated in the device and the power output of the power amplifier, and determines the optimal desired power to be output from the power amplifier. The closed-loop power control circuitry then delivers a power control signal to the power amplifier to control the power amplifier power output.
In a closed-loop power control system, the behavior of the power control loop, as well as overall system performance, depends to a large degree on the bandwidth of the power control loop and on the phase and gain margins of the power control loop.
Ideally, the bandwidth of the closed power control loop is maintained within a certain, and sometimes narrow, range to provide proper system functionality and to eliminate any out-of-band noise so as to comply with cellular transmission standards. One manner of calibrating a closed power control loop requires costly external components and dedicated test equipment. Such a calibration system is also time consuming.
Therefore, it would be desirable to minimize complexity and duration when calibrating a closed power control loop.